In recent years, a light-emitting panel composed of an organic EL element has been used as a lighting apparatus or a backlight of a liquid crystal display screen of an electronic information device. FIG. 1 shows a schematic constitution of an organic EL element used for a lighting apparatus or a backlight. In FIG. 1, a first electrode (hereinafter, referred to as an anode) 2 composed of a transparent electrode is formed on the top of a transparent substrate 1, and an organic layer 3 containing a luminescent compound is formed on the top of the anode 2. Furthermore, on the top of the organic layer 3, a second electrode (hereinafter, referred to as a cathode) 4 is formed. To the anode 2 and cathode 4, a light emission driving power source 6 is connected through a switch 7. When the switch 7 is turned on, electrons and positive holes are injected and recombined in the organic layer 3, thus excitons are generated. By use of emission of light (fluorescence, phosphorescence) when the excitons are deactivated, the organic layer 3 is permitted to emit light on the whole surface and it can be used as a lighting apparatus or a backlight. At this time, light can be emitted at a light emission driving voltage of several V to several tens V. Furthermore, the organic EL element has a wide view angle and a high visibility because it is a self-emission type, and the organic EL element also provides space saving because it is a film-type complete solid element. From these characteristics, the organic EL element is noticed as a light-emitting source (refer to Patent Document 1).
Further, for the purpose of lengthening the life span as a backlight, when the voltage of a battery power source drops, a method for emitting light from the half area is designed (refer to Patent Document 2).    Patent Document 1: Unexamined Japanese Patent Application No. 10-247401    Patent Document 2: Unexamined Japanese Patent Application No. 11-183901